audio-talk

Hi Nick See the edit above that I did to my post, at the same time as you were composing your reply. There's possibly a bit more evidence of the synergy you allude to in there though I can't prove any of what I'm positing. Using a power pentode to provide a CCS in an output stage might work far bett...

could do CCS on the output pairs but I very much do not want want to. Bloody solid state! - goes against the philosophy of it all and I think it (can, very easily, much too easily) degrades the sound. TBH that's what I thought could happen with too many transistors, which is why I decided in the en...

I look forward to hearing it some time. TBH, when I heard it at Steves (minus CCS), what was obvious (at least to me, maybe I was wrong), was a characteristic that I hear with most push pull amps, a sort of curdleing and thickening of the sound in the mid range, not sure if that description makes s...

Hi Guys and thanks for the comments. I think I'm starting to nail what it is that I like about the way this amp sounds, now it has the active circuitry in the tails of all the differential amp stages. It is all to do with the harmonic relationships, both within the envelope of an instrument and the ...

That'd be very useful Simon.
Thanks.

Steve

Here's the Jetson with Electro-Harmonix EL34s as constant current sinks

Those EL34s are much nicer looking than a pair of solid state devices bolted to heatsinks

Nice work Philip
well up to your usual high quality standards of build.

Steve.

I've been listening to the amp since 4pm today. Three and a half hours later, I still don't know what to say. I'm just totally and utterly gobsmacked! As you all know I'm never lost for words but I don't know how to begin to describe what I'm hearing. It's obviously not some kind of incredible best ...

Well I'll go to the foot of our stairs!

I thought you mentioned 70v on the cathodes before, thats when I said it was in pentode area. BTW, I had brainfade before, the cathode resistor will help things. Sorry about that. Hi Nick You really had me going last night. I got into real head-scratching territory :D You're right of course about t...

Hi Paul At the moment without the CCS I have 50V sitting at the top of each cathode resistor biasing the 6AS7s with 170V at each anode and 62mA quiescent current per section. The EL34 CCS: g2 is being supplied from a subsidiary HT supply left over from when I had the 12B4A preamp section. As a conse...

Here's the alternative This has the proper current ratios Ik to Ig2 and 10V of cathode bias so the valve follows the -10V line on the data sheet. Again it is worked out to give standard component values. The current through the valve is set by the 82R resistor. One advantage of using a pentode as a ...

OK

This is how it looks now

One thing though, I've just realised that when optimally biased, a pentode should have a ratio of 4:1 between cathode current and g2 current. Mine is nowhere near that.

120mA to 3.3mA is not 4:1 by any stretch of the imagination: more like 40:1.

Steve

Isn't having that cathode resistor going to defeat the point? The voltage will vary as the current through that varies. You could reduce it to 83 ohm if you took the grid to 0v, removing the 2M to B+. Or if you used a 9v battery to reverse bias the grid, you could loose the cathode resistor altoget...

OK I think I've worked out a pentode CCS for the output LTP using an EL34 as the sink. The 200V HT for the sink is provided by reusing the redundant supply still in there for the now defunct 12B4A preamp stage. I had to fiddle the currents (took quite a while messing about) so that the components wo...